Direct oxidative cyclization of 3-arylpropionic acids using PIFA or Oxone: synthesis of 3,4-dihydrocoumarins

Tetrahedron Letters 51 (2010) 192–196

Contents lists available at ScienceDirect

Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet

Direct oxidative cyclization of 3-arylpropionic acids using PIFA or Oxone: synthesis of 3,4-dihydrocoumarins Yonghong Gu *, Kun Xue Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China

a r t i c l e

i n f o

Article history: Received 2 July 2009 Revised 23 October 2009 Accepted 27 October 2009 Available online 30 October 2009

a b s t r a c t The direct oxidative cyclization of 3-arylpropionic acids using PIFA or Oxone is reported. In the presence of BF3OEt2, the reaction of 3-arylpropionic acids with PIFA or Oxone proceeded smoothly at 30 °C to give 3,4-dihydrocoumarins in good to excellent yields. Ó 2009 Elsevier Ltd. All rights reserved.

Coumarin and its derivatives are one of the most privileged structural motifs frequently found in natural products and pharmaceuticals.1 As an important class of coumarins, 3,4-dihydrocoumarins are widely distributed in nature2 and exhibit some interesting biological activities, such as anti-herpetic, anti-inflammatory, anti-oxidative, anti-aging, and anti-cancer activities.3 Therefore, development of efficient methods for the synthesis of dihydrocoumarins have attracted great interest in recent years.4 The conventional methods for the syntheses of dihydrocoumarins include (1) the transition-metal-catalyzed hydrogenation of coumarins;5 (2) Lewis or protonic acid-mediated hydroarylation of cinnamic acids with phenols;6 (3) the transition-metal-catalyzed intramolecular electrophilic hydroarylation of alkene substrates;7 (4) Lewis acid-mediated reaction of highly activated phenols with acrylonitriles,8 5-alkylidene Meldrum’s acids9 and hydroxyketeneS,S-acetals;10 (5) the Baeyer–Villiger oxidation of 1-indanones formed in situ by the Friedel–Crafts cyclization of 3-arylpropionic acids.11 However, many of these methods suffer disadvantages from using a large excess of expensive transition-metals, such as Pb(OAc)2,7f–h Ru(I),4c Ru(III),7e Y(OTf)3,6a Yb(OTf)3,9 Cr(CO)5,4e and harsh conditions. To achieve the environmental demands for ‘green’ chemical procedures, we were prompted to explore the feasibility of dihydrocoumarin synthesis by transition-metal-free oxidative C–O formation reaction from the corresponding carboxylic acids. Recently, utilization of hypervalent iodine compounds in oxidative reactions has received great attention for their low toxicity compared with heavy metal oxidants, mild reaction conditions, special reactivity, and easy handling.12 Several novel methods have been developed for the syntheses of lactones by intramolecular C–O bond formation using hypervalent iodine reagents.13 Kita and co-workers reported the cyclization of para-substituted

phenol14 or phenol ether derivatives15 to spiro dienone lactones using hypervalent iodine(III) reagents (Scheme 1). This method involves oxidation of phenols or phenol ethers to reactive electrophilic intermediates which are then trapped by carboxylic acid moiety. However, there is no report for the oxidative cyclization of electron-neutral or electron-deficient 3-arylylpropionic acids. We envisioned that oxidation of 3-arylylpropionic acids with hypervalent iodine reagents would generate radical cation intermediates, which could be attacked by carboxylic acid moiety to provide bicyclolactones. Herein, we disclose our investigation for the oxidative cyclization of 3-arylpropionic acids to 3,4dihydrocoumarins using hypervalent iodine reagents as oxidants. The initial attempts of the cyclization of 3-phenylpropionic acid (1a) with phenyliodine(III) diacetate (PIDA) or phenyliodine(III)

Kita's work: OR1

OR 1 R

2

R

O

2

R2

PIFA or OH PIDA

OH O O

O R1 = H, Me, Bn R 2 = H, Me, OMe, OBn

O

This work: R1

R1

OH O

R1

OH O

O O

1

R = electron-neutral or electron-withdrawing groups * Corresponding author. Tel.: +86 551 3602470; fax: +86 551 3601592. E-mail address: [email protected] (Y. Gu). 0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2009.10.112

Scheme 1. Cyclization of para-substituted phenol or phenol ether derivatives.

193

Y. Gu, K. Xue / Tetrahedron Letters 51 (2010) 192–196 Table 1 Optimization of reaction conditions using PIDA or PIFA as an oxidanta

O Oxidant(1.5 equiv) OH Additive(1.5 equiv) solvent, 30 oC, 12 h 1a Entry

Oxidant

1 2 3 4 5 6 7 8

PIDA PIDA PIDA PIDA PIFA PIDA PIFA PIFA

or or or or

PIFA PIFA PIFA PIFA

O 2a

O

Additive

Solvent

Yieldb (%)

None None None None None None BF3OEt2 BF3OEt2

CH2Cl2 CH3CN AcOH CF3CH2OH TFA TFA TFA TFA/CH2Cl2

NDc ND ND <5 17 <5 78 62d

a Unless otherwise specified, all the reactions were carried out in the presence of 0.2 mmol of 1a, 0.3 mmol of oxidant, and 0.3 mmol of additive in 1 mL of solvent at 30 °C for 12 h. b Isolated yield after column chromatography. c No desired product was detected by TLC analysis. d 0.2 mL of TFA and 1 mL of CH2Cl2 were used.

bis(trifluoroacetate) (PIFA) in CH2Cl2, CH3CN and AcOH did not give any 3,4-dihydrocoumarin (2a) at all, and no conversion of 1a was observed (Table 1, entries 1–3). When CF3CH2OH was used as a solvent, trace amount of 2a was obtained under otherwise identical conditions (entry 4). To our pleasure, when TFA was employed as a solvent, cyclization of 1a with PIFA provided the product 2a in 17% yield (entry 5). While the analogous reaction gave only trace amount of 2a with PIDA as an oxidant (entry 6). Notably, activation of PIFA with Lewis acid BF3OEt2 improved the reaction dramatically (entry 7).16 In addition, when CH2Cl2 (5:1 CH2Cl2/TFA) was used as co-solvent, the reaction afforded the product 2a in a slightly lower yield (entry 8). As a result, when the reaction was carried out in the presence of 1.5 equiv PIFA and 1.5 equiv BF3OEt2 in TFA at 30 °C, the best result was achieved. With optimized conditions in hand, next we examined the reaction scope and the results are summarized in Table 2. This reaction was compatible with a variety of functionalities, including methyl, phenyl, fluoride, bromide, iodide, and ester groups. The reaction of 3-(4-methylphenyl)propionic acid (1b) with PIFA provided the cyclized product 2b in 80% yield (entry 2). When the methyl group was introduced to the ortho position of phenyl ring, the reaction gave a slightly lower yield of 2c, probably due to steric effect (entry 3). When the methyl group was introduced to the meta position of phenyl ring, two products 2da and 2db were obtained in 82% yield as a 2:1 mixture with a less sterically hindered product 2da as the major (entry 4).17 When the para position of phenyl ring was substituted with halogens (F, Br, I), the reaction generally proceeded well to give corresponding products 2e–g in 54–79% yields (entries 5–7). It is noteworthy that when the substrate with an electron-withdrawing group, ester group at the para position of phenyl ring was employed, the reaction gave the cyclized product 2h in a slightly lower yield for a longer reaction time (entry 8). Nevertheless, when the electrondonating group, methoxy group, was introduced to the para position of phenyl ring, the reaction failed to yield any desired product 2i; instead, a complex mixture was obtained (entry 9).18 To expand the substrate scope, we examined the substitution effect on the chain of 3-arylpropionic acid. It was found that when the substrates with methyl or phenyl group at the b-position of the carboxylic acid were employed, the reaction proceeded smoothly to give the products 2j and 2k in 90% and 75% yields,

respectively (entries 10 and 11). Notably, when the substrate contains the hydroxyl group at the b-position of the carboxylic acid, the reaction gave the dehydrated product 2l in moderate yield (entry 12). To make this reaction more synthetically applicable, we extended our research to seek low-cost and ‘greener’ oxidants. Peroxides as inexpensive, environmentally benign oxidants are extensively used in modern organic synthesis. Due to their prominent properties, several peroxides were explored in the oxidative cyclization of 3-phenylpropionic acid (1a) (Table 3). It was found that peroxides, such as m-CPBA, H2O2, and t-BuOOH failed to provide any desired product 2a in this reaction (entries 1–3). To our delight, both K2S2O8 and Oxone were effective to this transformation. In the presence of 2.0 equiv oxidants and 1.5 equiv of BF3OEt2 in TFA at 30 °C, the reactions provided 2a in 42% and 61% yields, respectively (entries 4 and 5). In addition, Lewis acid BF3OEt2 is also essential in this transformation (entries 5 and 6). This result indicated that BF3OEt2 probably activated the epoxide intermediate 8 formed in the reaction and facilitated this transformation (Scheme 2). Careful examination of the solvent showed TFA is the ideal solvent (entries 7–9). Therefore, using Oxone19 as an oxidant and BF3OEt2 as an additive in TFA were chosen as desired reaction conditions. In the presence of 2.0 equiv Oxone and 1.5 equiv BF3OEt2, the reaction of acids 1a–l in TFA proceeded smoothly to provide cyclized products 2a–l in 16–71% yields (Table 2). This reaction proceeded generally comparable or moderately lower yields compared with PIFA-mediated reaction. This transformation could be applied to the substrates containing various functional groups, including methyl, phenyl, halides, and methoxy groups (entries 2–11). Notably, in some cases, Oxone showed the improved functional group tolerance relative to PIFA. For example, the cyclization of methyl ether 1i provided 16% yield of 2i, the analogous reaction failed to yield 2i with PIFA as an oxidant (entry 9). However, when the substrate contains an electron-withdrawing group, (e.g., in 3-(4-carbomethoxyphenyl)propionic acid (1h)), the reaction was not successful (entry 8). PIFA or Oxone (1.5-2.0 equiv)

3

O

BF3. OEt2 (1.5 equiv)

O

TFA, 30 oC, 24 h

2a

O

ð1Þ

It should be noteworthy that the oxidation of 1-indanone (3) with PIFA or Oxone in the presence of BF3OEt2 in TFA did not provide 3,4-dihydrocoumarin (2a) (Eq. 1). Therefore, the formation of 1-indanone from the 3-phenylpropionic acid with TFA followed by the oxidation to dihydrocoumarin should be excluded. Two plausible path ways are proposed according to the literature examples and our above experimental results (Scheme 2).15c,20 First, the one electron oxidation of aromatic ring of 1 by PIFA forms the radical cation 4. Then intramolecular nucleophilic attack by the carboxylic acid moiety on radical cation yields the bicyclolactone cation 5 (path a) or spiro lactone cation 6 (path b); rearrangement of the carboxylic unit of 6 leads to intermediate 5, the deprotonation of 5 generates dihydrocoumarin 2. Additionally, the reaction of phenol derivatives (R1 = OH) with PIFA and BF3OEt2 at 78 °C afforded the spiro dienone lactone 7 in 57% yield. This result suggests that the reaction may proceed via the rearrangement pathway. Furthermore, we also proposed the mechanism of the oxidative cyclization by Oxone.21 Oxidation of 1 by Oxone forms the epoxide 8, followed by intramolecular nucleophilic attack of epoxide with carboxylic acid moiety to provide the intermediate 9. Last dehydration of 9 furnishes dihydrocoumarin 2.

194

Y. Gu, K. Xue / Tetrahedron Letters 51 (2010) 192–196

Table 2 Reaction of 3-arylpropionic acid with PIFA or Oxonea

R2

OH

R1

1

Entry

R2

O

Substrate

PIFA (1.5 equiv) or Oxone (2.0 equiv)

R1

BF3.OEt 2 (1.5 equiv) TFA, 30 o C, 12-24 h

O

O

2

Product

Time (h)

Yieldb (%) PIFA/Oxone

12

78/61

12

80/61

16

63/44

12

82/61(2:1)

16

54/49

12

79/58

12

65/56

24

50/NDc

12

ND/16

O OH

1

O

O

2a 1a O OH

2

O

O

2b 1b O OH

3

O

O

2c

1c O OH

O

O

2da 4

1d O

O

2db O OH

5

F

O

O

2e

F

1e O OH

6

Br

O

O

2f

Br

1f O OH

7

I

I

O

O

2g 1g O OH

8

MeO 2C

MeO 2C

O

O

2h 1h O OH

9

MeO

MeO

O

2i 1i

O

195

Y. Gu, K. Xue / Tetrahedron Letters 51 (2010) 192–196 Table 2 (continued) Entry

Substrate

Product

Time (h)

Yieldb (%) PIFA/Oxone

12

90/71

12

75/66

16

48/51

O OH

10

O

O

2j

1j Ph

Ph

O OH

11

1k

2k

OH

O

O

O

O

O OH

12

2l 1l a b c

Reactions were carried out with 0.2 mmol of 1, 0.3 mmol of PIFA or 0.4 mmol of Oxone, 0.3 mmol of BF3OEt2 in 1 mL of TFA at 30 °C. Isolated yield after column chromatography. No desired product was detected by TLC analysis.

Table 3 Optimization of reaction conditions using peroxide as an oxidanta

O Oxidant(2.0 equiv) OH

Additive(1.5 equiv) solvent, 30 oC, 12 h

O 2a

1a

O

Entry

Oxidant

Additive

Solvent

Yieldb (%)

1 2 3 4 5 6 7 8 9

m-CPBA H2O2 t-BuOOH K2S2O8 Oxone Oxone Oxone Oxone Oxone

BF3OEt2 BF3OEt2 BF3OEt2 BF3OEt2 BF3OEt2 None BF3OEt2 BF3OEt2 BF3OEt2

TFA TFA TFA TFA TFA TFA CH2Cl2 CF3CH2OH AcOH

NDc ND <5 42 61 35 ND ND ND

In conclusion, we developed a novel method for the synthesis of 3,4-dihydrocoumarin through direct oxidative cyclization of 3-arylpropionic acids using PIFA or Oxone as an oxidant. The synthetic utility of these reactions is enhanced by using Oxone as an inexpensive, safe, and environmentally benign oxidant. Ongoing work to expand the substrate scope and apply this reaction to the synthesis of more complex products is underway. Acknowledgments We thank the financial support for this work from the National Natural Science Foundation of China (No. 20602033) and National Basic Research Program of China (2006CB922001). Supplementary data

a

Unless otherwise specified, all the reactions were carried out in the presence of 0.2 mmol of 1a, 0.4 mmol of oxidant, and 0.3 mmol of additive in 1 mL of solvent at 30 °C for 12 h.

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.2009.10.112. References and notes

PIFA R1

HO 1

O

O

Path b

BF 3.OEt2

R1

HO

O

O

R1 6

4

-78 oC Path a O O R1

O

O

R1

O

5

2

Oxone HO 1

H

O

. O BF3 OEt2

HO

O

O

7 (R 1 = OH) 57%

OCOCF 3

OH

BF 3.OEt 2

2

O H

8 9

O

O OCOCF 3

Scheme 2. Proposed mechanism of lactonization of 3-arylpropionic acid.

1. (a) O’Kennedy, R.; Thomes, R. D. Coumarins: Biology, Applications and Mode of Action; Wiley & Sons: Chichester, UK, 1997; (b) Murray, R. D. H.; Mendez, J.; Brown, S. A. The Natural Coumarins: Occurrence, Chemistry and Biochemistry; Wiley & Sons: New York, 1982; (c) Naser-Hijazi, B.; Stolze, B.; Zanker, K. S. Second Proceedings of the International Society of the Coumarin Investigators; Springer: Berlin, Germany, 1994; (d) Yu, D.; Suzuki, M.; Xie, L.; MorrisNatschke, S. L.; Lee, K.-H. Med. Res. Rev. 2003, 23, 322. 2. (a) Donnelly, D. M. X.; Boland, G. M. Nat. Prod. Rep. 1995, 321; (b) Donnelly, D. M. X.; Boland, G. M. In The Flavonoids: Advances in Research Since 1986; Harborne, J. B., Ed.; Chapman and Hall: London, 1993. Chapter 6; (c) Matern, U.; } er, P.; Kreusch, D.. In Comprehensive Natural Products Chemistry; Sankawa, U., Lu Ed.; Pergamon: Oxford, 1999; Vol. 1, p 623. 3. (a) Kumar, A.; Singh, B. K.; Tyagi, R.; Jain, S. K.; Sharma, S. K.; Prasad, A. K.; Raj, H. G.; Rastogi, R. C.; Watterson, A. C.; Parmar, V. S. Bioorg. Med. Chem. 2005, 13, 4300; (b) Roelens, F.; Huvaere, K.; Dhooge, W.; Cleemput, M. V.; Comhaire, F.; Keukeleire, D. D. Eur. J. Med. Chem. 2005, 40, 1042; (c) Posakony, J.; Hirao, M.; Stevens, S.; Simon, J. A.; Bedalov, A. J. Med. Chem. 2004, 47, 2635; (d) Zhang, X.; Wang, H.; Song, Y.; Nie, L.; Wang, L.; Liu, B.; Shen, P.; Liu, Y. Bioorg. Med. Chem. Lett. 2006, 16, 949. 4. For recent examples of syntheses of dihydrocoumarins, see: (a) Zeitler, K.; Rose, C. A. J. Org. Chem. 2009, 74, 1759; (b) Shaabani, A.; Soleimani, E.; Rezayan, A. H.; Sarvary, A.; Khavasi, H. R. Org. Lett. 2008, 10, 2581; (c) Matsuda, T.; Shigeno, M.; Murakami, M. J. Am. Chem. Soc. 2007, 129, 12086; (d) Girotti, R.; Marrocchi, A.; Minuti, L.; Piermatti, O.; Pizzo, F.; Vaccaro, L. J. Org. Chem. 2006, 71, 70; (e) Barluenga, J.; Andina, F.; Aznar, F. Org. Lett. 2006, 8, 2703; (f) Ye, M.-C.; Yang, Y.Y.; Tang, Y.; Sun, X.-L.; Ma, Z.; Qin, W.-M. Synlett 2006, 8, 1240.

196

Y. Gu, K. Xue / Tetrahedron Letters 51 (2010) 192–196

5. Noyori, R. Asymmetric Catalysis in Organic Synthesis; John Wiley and Sons: New York, 1994; (b) McGuire, M. A.; Shilcrat, S. C.; Sorenson, E. Tetrahedron Lett. 1999, 40, 3293. 6. (a) Rodrigues-Santos, C. E.; Echevarria, A. Tetrahedron Lett. 2007, 48, 4505; (b) Jagdale, A. R.; Sudalai, A. Tetrahedron Lett. 2007, 48, 4895; (c) Krawczyk, H.; Albrecht, L.; Wojciechowski, J.; Wolf, W. M. Tetrahedron 2007, 63, 12583. 7. (a) Oyamada, J.; Kitamura, T. Tetrahedron 2006, 62, 6918; (b) Li, K.; Foresee, L. N.; Tunge, J. A. J. Org. Chem. 2005, 70, 2881; (c) Aoki, S.; Amamoto, C.; Oyamada, J.; Kitamura, T. Tetrahedron 2005, 61, 9291; (d) Song, C. E.; Jung, D.; Choung, S. Y.; Roh, E. J.; Lee, S. Angew. Chem., Int. Ed. 2004, 43, 6183; (e) Youn, S. W.; Pastine, S. J.; Sames, D. Org. Lett. 2004, 6, 581; (f) Shi, Z.; He, C. J. Org. Chem. 2004, 69, 3669; (g) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102, 1731; (h) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem. Res. 2001, 34, 633; (i) Jia, C.; Piao, D.; Oyamada, J.; Lu, W.; Kitamura, T.; Fujiwara, Y. Science 2000, 287, 1992; (j) Jia, C.; Lu, W.; Oyamada, J.; Kitamura, T.; Matsuda, K.; Irie, M.; Fujiwara, Y. J. Am. Chem. Soc. 2000, 122, 7252. 8. Johnston, K. M. Tetrahedron 1968, 24, 5595. 9. Fillion, E.; Dumas, A. M.; Kuropatwa, B. A.; Malhotra, N. R.; Sitler, T. C. J. Org. Chem. 2006, 71, 409. 10. Piao, C.-R.; Zhao, Y.-L.; Han, X.-D.; Liu, Q. J. Org. Chem. 2008, 73, 2264. 11. (a) Poras, H.; Stephan, E.; Pourcelot, G.; Cresson, P. Chem. Ind. 1993, 206; (b) Patra, A.; Misra, S. K. Indian J. Chem. 1990, 29B, 66; (c) Scott, A. I.; Dodsox, P. A.; McCapra, F.; Meyers, M. B. J. Am. Chem. Soc. 1963, 85, 3702. 12. For recent reviews of hypervalent iodine reagents in oxidation reaction, see: (a) Uyanik, M.; Ishihara, K. Chem. Commun. 2009, 2086; (b) Dohi, T.; Kita, Y. Chem. Commun. 2009, 2073; (c) Zhdankin, V. V.; Stang, P. J. Chem. Rev. 2008, 108, 5299; (d) Ochiai, M.; Miyamoto, K. Eur. J. Org. Chem. 2008, 4229; (e) Zhdankin, V. V. Sci. Synth. 2007, 31a, 161. chapter 31.4.1; (f) Wirth, T. Angew. Chem., Int. Ed. 2005, 44, 3656; (g) Moriarty, R. M. J. Org. Chem. 2005, 70, 2893; (h) Tohma, H.; Kita, Y. Adv. Synth. Catal. 2004, 346, 111; (i)Hypervalent Iodine Chemistry;

13. 14.

15.

16.

17. 18. 19.

20. 21.

Wirth, T., Ed.; Springer: Berlin, 2003; (j) Zhdankin, V. V.; Stang, P. J. Chem. Rev. 2002, 102, 2523. (a) Dohi, T.; Takenaga, K.; Goto, A.; Maruyama, A.; Kita, Y. Org. Lett. 2007, 9, 3129; (b) Browne, D. M.; Niyomura, O.; Wirth, T. Org. Lett. 2007, 9, 3169. (a) Tamura, Y.; Yakura, T.; Haruta, J.; Kita, Y. J. Org. Chem. 1987, 52, 3927; (b) Ley, S. V.; Thomas, A. W.; Finch, H. J. Chem. Soc., Perkin Trans. 1 1999, 669; (c) Kita, Y.; Tohma, H.; Kikuchi, K.; Inagaki, M.; Yakura, T. J. Org. Chem. 1991, 56, 435; (d) Hara, H.; Inoue, T.; Nakamura, H.; Endoh, M.; Hoshino, O. Tetrahedron Lett. 1992, 33, 6491. (a) Dohi, T.; Maruyama, A.; Takenaga, N.; Senami, K.; Minamitsuji, Y.; Fujioka, H.; Caemmerer, S. B.; Kita, Y. Angew. Chem., Int. Ed. 2008, 47, 3787; (b) Dohi, T.; Maruyama, A.; Yoshimura, M.; Morimoto, K.; Tohma, H.; Kita, Y. Angew. Chem., Int. Ed. 2005, 44, 6193; (c) Hata, K.; Hamamoto, H.; Shiozakr, Y.; Kita, Y. Chem. Commun. 2005, 2465. Lewis acid BF3OEt2 was often used in PIFA-mediated oxidative coupling reaction which usually improved the reaction. It was believed that trifluoroacetoxy ligand of PIFA might coordinate to BF3OEt2 and generate a reactive cationic iodine(III) intermediate. See reference: Dohi, T.; Ito, M.; Morimoto, K.; Iwata, M.; Kita, Y. Angew. Chem., Int. Ed. 2008, 47, 1301 and references cited therein. The ratio of products 2da and 2db was determined from the mixture of the products by 1H NMR spectrum. When the reaction of 1i with PIFA and BF3OEt2 in TFA was carried out at 78 °C, spiro dienone lactone 7 was obtained in 40% yield. For recent examples of Oxone in oxidative reaction, see: (a) Ho, C. Y.; Chen, Y. C.; Wong, M. K.; Yang, D. J. Org. Chem. 2005, 70, 898; (l) Pezzella, A.; Palma, A.; Iadonisi, A.; Napolitano, A.; d’Ischia, M. Tetrahedron Lett. 2007, 48, 3883. Hamamoto, H.; Hata, K.; Nambu, H.; Shiozaki, Y.; Tohma, H.; Kita, Y. Tetrahedron Lett. 2004, 45, 2293. Yang, D.; Wong, M.-K.; Yan, Z. J. Org. Chem. 2000, 65, 4179.